Optical communication system having optical amplification function

ABSTRACT

In optical communications between a base station and a local station, the wavelength of a laser light source for a signal, high LD, in the base station that generates downstream signal light is set to a wavelength with an effect of Raman amplifying an upstream light signal that propagates through an optical fiber  2.  In the optical fiber  2,  an upstream light signal transmitted from the local station to the base station is amplified with the downstream signal light from the laser light source for a signal, high LD, while the upstream light signal is propagating through the optical fiber  2.

FIELD OF THE INVENTION

The present invention relates to an optical communications system inwhich a base station and a local station are connected using an opticalfiber.

The invention relates to an optical communications system, and moreparticularly, to a PON (Passive Optical Network) system in which a basestation and an optical branching station equipped with a passive opticaldivider are connected using a backbone optical fiber, and the opticalbranching station and plural local stations are connected individuallyusing branch optical fibers.

BACKGROUND ART

In a system enabling two-way communications between a base station andplural local stations using an optical data communications network, anetwork configuration (Single Star) connecting the base station and therespective local stations in a radial pattern using a single opticalfiber for each local station is now put into practical use. With thisnetwork configuration, the system and the device configuration can besimpler; however, because each local station occupies a single opticalfiber, it is difficult to reduce the cost of the system.

Such being the case, a PON (Passive Optical Network) system (referred toalso as PDS (Passive Double Star)), in which a single optical fiber isshared among plural local stations, has been proposed. In the PONsystem, the base station and the optical branching station equipped witha passive optical divider are connected using a backbone optical fiber,and the optical branching station and plural local stations areindividually connected using branch optical fibers.

In the PON system, in order to ensure power needed for opticaltransmission, a configuration to amplify a light signal travelingthrough the optical fiber by incorporating an optical amplifier into theoptical branching station has been proposed (see Japanese UnexaminedPatent Publication No. 9-181686 (1997)A).

The configuration as above, however, has problems that the use of theoptical amplifier in the optical branching station increases the costfor the purchase and installment, and that maintenance takes time andlabor because a technical person has to go to the optical branchingstation in the event of trouble after installment.

Also, besides the PON system, an optical amplifier is inserted to theoptical fibers between the base station and plural local stations in anormal optical communications system. However, there are problems thatthe use of the optical amplifier increases the cost for the purchase andinstallment, and that maintenance takes time and labor because atechnical person has to go to the site where optical amplifier isinstalled in the event of trouble after installment.

Hence, if one succeeds in distributing and furnishing the amplificationfunction to optical fibers instead of using an optical amplifier as asingle item, the maintenance can be easier and-a reduction of the costcan be expected due to mass-production.

DESCRIPTION OF THE INVENTION

The invention therefore has an object to provide an opticalcommunications system capable of furnishing optical fibers with theoptical amplification function.

An optical communications system of the invention is characterized inthat a wavelength of a light source for a signal that generatesdownstream signal light is set to a wavelength with an effect of Ramanamplifying an upstream light signal that propagates through an opticalfiber, and an upstream light signal transmitted between a base stationand a local station is amplified in the optical fiber while the upstreamlight signal is propagating through the optical fiber.

According to this configuration, light for a signal having a wavelengthwith an effect of amplifying an upstream light signal is generated usingthe light source for a signal, and the light for a signal is transmittedto the local station via the optical fiber. It is thus possible toamplify upstream signal light traveling through the optical fiber withease. The base station and the local station can be chosen arbitrarily,and either station equipped with a light source for a signal having awavelength with the Raman amplification effect can be used as the basestation.

FIG. 15 is a graph showing the conditions of the Raman amplification,using the abscissa for a wavelength and the ordinate for optical powerduring propagation. Assume that signal light and light for amplificationpropagate in directions opposite to each other. In order to perform theRaman amplification, it is sufficient for the wavelength of light foramplification to be about 0.1 μm shorter than the wavelength of signallight.

Further, as the amplification conditions, it is preferable that theRaman gain, (gR/Aeff)PpLeff, is 0.1 dB or higher, where (gR/Aeff) is aRaman gain coefficient of the optical fiber, Pp is pumping powerinputted into the optical fiber, and Leff is an effective distance alongthe optical fiber over which pumping light functions.

It is preferable that a high nonlinearity fiber is used for at leastpart of the optical fiber (Claim 2). The high nonlinearity fiberreferred to herein is defined as an optical fiber having the Raman gain,(gR/Aeff)PpLeff, of 4 dB or higher. For example, it can be manufacturedby slightly reducing the core diameter from that of a general singlemode optical fiber. Because a high nonlinearity effect can be achievedwith the use of the high nonlinearity fiber, the amplification gain of alight signal can be set high. It is thus possible to amplify an upstreamsignal even when the light source for a signal that generates downstreamsignal light has relatively low power or the distance is short. The term“at least part of” is used because the high nonlinearity fiber does nothave to be used for the entire transmission path, and it is sufficientto use the high nonlinearity fiber for a distance long enough to obtaina needed amplification gain. For example, in the case of long distancetransmission, it is effective to connect the high nonlinearity fiber andan SMF (Single Mode Fiber) in series while forming a portion closer tothe light source for a signal in the base station using the highnonlinearity fiber and a remote portion using the SMF.

Light that is switched ON and OFF may be used as the downstream signallight, and a modulation method, by which an ON state and an OFF statetransit even when coded data is a sequence of 0's and the ON state andthe OFF state transit even when the coded data is a sequence of 1's, maybe used as a modulation method for the downstream signal light (Claim3). When configured in this manner, fluctuation of the amplificationgain can be suppressed because the ON state does not continue for a longperiod and the OFF state does not continue for a long period, either,which enables a stable amplification characteristic to be achieved. Inparticular, this is effective in suppressing fluctuation of theamplification gain when a ratio of the ON state and the OFF state isconstant.

It is preferable that, in the optical fiber, a length of a portion whereupstream signal light is amplified is of a distance longer than a lengthof the optical fiber corresponding to a set of the ON state and the OFFstate of the downstream signal light (Claim 4). For example, assume thata light signal propagates an optical fiber having a given length L (m)at a rate, c/n (m/sec), where c is a rate of light in vacuum and n is aneffective refractive index of the optical fiber. Given A (bits/sec) asthe transmission rate of a signal when an encoding method, by which abits are transmitted by one set of an ON state and an OFF state onaverage, is used, then nLA/αc sets of an ON state and an OFF state arepresent in the optical fiber having the length L (m). Because a signallight is present in about half the sets of an ON state and an OFF stateof downstream signal light, by making the length L (m) of the opticalfiber longer than αc/nA (m), it is possible to perform the stable Ramanamplification over the length L (m) of the optical fiber.

It is preferable that, in the base station, an optical filter used toselect a wavelength of light coming incident on a light-receivingelement is provided (Claim 5).

A PON system of the invention is characterized in that a wavelength of alight source for a signal that generates downstream signal light is setto a wavelength with an effect of Raman amplifying an upstream lightsignal that propagates through a backbone optical fiber, and an upstreamlight signal transmitted between a base station and a local station isamplified in the backbone optical fiber while the upstream light signalis propagating through the backbone optical fiber (Claim 6).

According to the configuration as above, light for a signal having awavelength with an effect of amplifying an upstream light signal isgenerated using the light source for a signal, and the light for asignal is distributed to local stations via a backbone optical fiber andby way of an optical multiplexer/demultiplexer. It is thus possible toamplify upstream signal light traveling through the backbone opticalfiber with ease.

Because the Raman amplification is used as the function of amplifying alight signal, it is possible to distribute and amplify upstream signallight traveling through the optical fiber by allowing propagation oflight for a downstream signal. As has been described, by furnishing theoptical amplification function to the optical fiber, the need to preparethe optical amplifier in an optical branching station can be eliminated.A PON system of a simple configuration can be thus achieved.

It is preferable that a high nonlinearity fiber is used for at leastpart of the backbone optical fiber (Claim 7). Because a highnonlinearity effect can be achieved with the use of the highnonlinearity fiber, a high gain can be obtained with relatively weakamplifying light. Optical power of the light source for a signal may betherefore relatively low. In the case of long distance transmission, itis more effective to connect the high nonlinearity fiber and an SMF(Single Mode Fiber) in series while forming a portion closer to thelight source for a signal in the base station using the highnonlinearity fiber and a remote portion using the SMF.

In the case above, by using a modulation method, by which an ON stateand an OFF state transit even when coded data is a sequence of 0's andthe ON state and the OFF state transit even when coded data is asequence of 1's, as a modulation method for switching ON/OFF thedownstream signal light (Claim 8), the Raman amplification can beperformed on a light signal in an ON state. This enables the stableamplification characteristic to be achieved. When a method forsubjecting signal light to polarization modulation or phase modulationis used, stable amplification can be performed constantly without havingto concern the coding method, because optical power hardly varies withtime.

Also, in order to achieve a stable amplification characteristic, it ispreferable that, in the backbone optical fiber, a length of a portionwhere upstream signal light is amplified is of a distance longer than alength of the backbone optical fiber corresponding to a set of the ONstate and the OFF state of the downstream signal light (Claim 9).

A concrete configuration of the PON system of the invention will now bedescribed. Figure numbers inside the parentheses indicate correspondingfigure numbers used in the descriptions of embodiments below.

As the configuration of the PON system of the invention, by providingthe light source for a signal and an optical multiplexer/demultiplexerin the base station, and by pumping light for a signal into the backboneoptical fiber from the base station toward the optical branching stationby way of the optical multiplexer/demultiplexer, it is possible toamplify an upstream light signal between the base station and theoptical branching station (Claim 10). Because a light signal from thelocal station travels over a long propagation path, and a distancebetween the base station and the optical branching station is long inmany cases, it is effective to amplify the light signal over thisdistance.

In this system configuration, a star coupler can be used as a passiveoptical divider (Claim 11, FIG. 4). According to this configuration, themanufacturing and management costs can be saved by using an inexpensivestar coupler. Also, because all the local stations can handle a lightsignal of the same wavelength, the manufacturing costs of the localstations can be reduced.

Also, in this system configuration, as the passive optical divider, astar coupler can be used for the downstream signal light, and an AWGcapable of multiplexing and demultiplexing upstream signal light using adifference in wavelength can be used for the upstream signal light(Claim 12, FIG. 5). By using the AWG for an upstream signal, theupstream signal light can be multiplexed and demultiplexed at a smallloss. This provides allowance to the optical power design regarding alight source for a signal in the local station.

Also, a PON system of the invention includes: a light source foramplification that generates light for amplification having a wavelengthwith an effect of amplifying a light signal propagating through anoptical fiber (including a backbone optical fiber and a branch opticalfiber); and an optical multiplexer/demultiplexer used to pump the lightfor amplification into the optical fiber. In the optical fiber, a lightsignal transmitted between a base station and a local station isamplified while the light signal is propagating through the opticalfiber (Claim 13).

According to the configuration above, light for amplification having awavelength with an effect of amplifying a light signal is generatedusing the light source for amplification, and the light foramplification is pumped into the optical fiber by way of the opticalmultiplexer/demultiplexer. It is thus possible to amplify the signallight traveling through the optical fiber with ease.

When the Raman amplification is used as a function of amplifying a lightsignal, by allowing the light for amplification to propagate in adirection opposite to the signal light (Claim 14), it is possible todistribute and amplify the signal light traveling through the opticalfiber.

As an optical fiber achieving the Raman amplification, a highnonlinearity fiber can be used (Claim 15). Because a high nonlinearityeffect can be achieved with the use of the high nonlinearity fiber, ahigh gain can be obtained with relatively weak amplifying light. In thecase of long distance transmission, it is more effective to connect thehigh nonlinearity fiber and an SMF (Single Mode Fiber) while forming aportion closer to the light source for amplification using the highnonlinearity fiber and a remote portion using the SMF.

Besides the Raman amplification, when an erbium-doped fiber (EDF) isused as a function of amplifying the light signal (Claim 16), it ispossible to amplify signal light in the same direction as the signal foramplification through the use of induced emission of erbium ions.

In the cases above, by using non-modulated light as the light foramplification, a further stable amplification characteristic can beachieved.

By providing the light source for amplification and the opticalmultiplexer/demultiplexer in the base station, and by pumping the lightfor amplification into the backbone optical fiber from the base stationtoward the optical branching station, it is possible to amplify a lightsignal between the base station and the optical branching station (Claim17, FIG. 6). Because a light signal from the local station travels overa long propagation path, and a distance between the base station and theoptical branching station is long in many cases, it is effective toamplify the light signal over this distance.

By providing the light source for amplification and the opticalmultiplexer/demultiplexer in the optical branching station, and bypumping the light for amplification into the backbone optical fiber fromthe optical multiplexer/demultiplexer toward the base station, it ispossible to amplify a light signal between the base station and theoptical branching station (Claim 18, FIG. 7).

In addition to the configuration set forth in claim 17, by providing asecond optical multiplexer/demultiplexer, a third opticalmultiplexer/demultiplexer, and an optical path connecting the secondoptical multiplexer/demultiplexer and the third opticalmultiplexer/demultiplexer in the optical branching station, and byextracting the light for amplification that travels through a backboneoptical fiber for an upstream signal from the second opticalmultiplexer/demultiplexer to be supplied to the third opticalmultiplexer/demultiplexer via the optical path, it is possible to pumpthe light for amplification into a backbone optical fiber for adownstream signal from the third optical multiplexer/demultiplexertoward the base station (Claim 19, FIG. 8).

According to this configuration, by pumping light for amplificationtraveling through the backbone optical fiber for an upstream signal fromthe base station again into the backbone optical fiber for a downstreamsignal toward the base station, it is possible to amplify downstreamsignal light. By setting the wavelengths of both the upstream signallight and the downstream signal light to the same wavelength, both theupstream and downstream signals can be amplified efficiently by a singlelight source for amplification.

A configuration, in which the light source for amplification and theoptical multiplexer/demultiplexer are provided in the optical branchingstation, so that the light for amplification is pumped into the branchoptical fiber by way of the passive optical divider toward the localstation, may be adopted (Claim 20, FIG. 9). When configured in thismanner, a light signal between the optical branching station and thelocal station can be also amplified.

Also, by providing the light source for amplification and the opticalmultiplexer/demultiplexer in the base station, and by pumping the lightfor amplification into the backbone optical fiber from the base stationtoward the optical branching station, while providing a reflector thatallows the light for amplification to undergo total reflection to thebackbone optical fiber in the optical branching station (Claim 21, FIG.10), it is possible to amplify a light signal using the light source foramplification provided in the base station without having to provide thelight source for amplification in the optical branching station. Thereflector can be achieved, for example, by an FBG (Fiber Bragg Grating).

A configuration, in which the light source for amplification and theoptical multiplexer/demultiplexer are provided in the base station forthe light for amplification to be pumped into the backbone optical fiberfrom the base station toward the optical branching station, while asecond optical multiplexer/demultiplexer and a reflector are provided inthe optical branching station for the light for amplification thattravels through the backbone optical fiber to be extracted from thesecond optical multiplexer/demultiplexer, so that the light foramplification is allowed to undergo total reflection on the reflector(Claim 22, FIG. 11), may be adopted. It is thus possible to amplify alight signal using the light source for amplification provided in thebase station without having to provide the light source foramplification in the optical branching station.

A configuration, in which the optical multiplexer/demultiplexer isprovided in the optical branching station and an optical fiber isprovided between the base station and the optical branching stationbesides the backbone optical fiber, while the light source foramplification is provided in the base station for the light foramplification to be supplied to the optical multiplexer/demultiplexervia the optical fiber, so that the light for amplification is pumpedinto the backbone optical fiber from the opticalmultiplexer/demultiplexer toward the base station (Claim 23, FIG. 12),is also possible. According to this configuration, the need to providethe light source for amplification in the optical branching station canbe eliminated by providing the optical fiber between the base stationand the optical branching station. It is thus possible to maintain andmanage the light source for amplification with ease. Also, operations ofthe optical multiplexer/demultiplexer can be obtained from the passiveoptical divider.

In the system configurations set forth in claim 17 through claim 23, astar coupler can be used as the passive optical divider (Claim 24). Themanufacturing and the management costs can be saved by using aninexpensive star coupler.

A configuration, in which an optical fiber is provided between the basestation and the optical branching station besides the backbone opticalfiber, and the light source for amplification is provided in the basestation, so that the light for amplification is pumped into one opticalpath of the optical multiplexer/demultiplexer on the local station sidevia the optical fiber toward the base station (Claim 25, FIG. 13), isalso possible.

According to this configuration, the need to provide the light sourcefor amplification in the optical branching station can be eliminated byproviding the optical fiber between the base station and the opticalbranching station. It is thus possible to maintain and manage the lightsource for amplification with ease. Also, operations of the opticalmultiplexer/demultiplexer can be obtained from the passive opticaldivider. Hence, there is no need to prepare an opticalmultiplexer/demultiplexer other than the passive optical divider, whichmakes the configuration of the optical branching station simpler.

In the system configurations set forth in claim 17 through claim 25(excluding claim 24), an AWG capable of multiplexing and demultiplexinglight having different wavelengths can be used in the optical branchingstation (Claim 26). By using the AWG, amplifying light can be separatedat a small loss.

As has been described, according to the invention, the need to preparethe optical amplifier in the optical branching station can be eliminatedby furnishing the optical fiber with the optical amplification function.A PON system of a simple configuration can be thus achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an optical communications systemfurnished with an optical amplification function of the invention;

FIG. 2 is a network configuration view showing a state where an opticalline terminal OLT in a base station 1 and an optical network unit ONU ina local station 5 are connected to each other;

FIG. 3 is a block diagram showing an overall PON system furnished withan optical amplification function of the invention;

FIG. 4 is a view showing the configuration of a PON system of theinvention that amplifies an upstream signal propagating through abackbone optical fiber using a High LD for a signal in the base station;

FIG. 5 is a view showing the configuration of a PON system of theinvention using a star coupler for downstream signal light and an AWGfor upstream signal light to multiplex/demultiplex signal light;

FIG. 6 is a view showing the configuration of a PON system of theinvention that amplifies an upstream signal propagating through backboneand branch optical fibers by providing an LD for amplification in thebase station;

FIG. 7 is a view showing the configuration of a PON system of theinvention that amplifies a downstream signal from the base station byproviding an LD for amplification also in the optical branching station;

FIG. 8 is a view showing the configuration of a PON system of theinvention capable of amplifying an upstream signal to the base stationand a downstream signal from the base station by merely providing one LDfor amplification in the base station;

FIG. 9 is a view showing the configuration of a PON system, including anadditional configuration to the configuration of FIG. 7, that amplifiesan upstream light signal from the local station to the base station withlight from an LDb for amplification provided in the optical branchingstation;

FIG. 10 is a view showing the configuration of a PON system of theinvention in which LD 2 and LD 3 for amplification are provided in thebase station, so that a downstream signal propagating through a backboneoptical fiber can be amplified with light from the LD 2, and an upstreamsignal propagating through the backbone and the branch optical fiberscan be amplified with light from the LD 3;

FIG. 11 is a view showing the configuration of a PON system of theinvention in which LD 2 and LD 3 for amplification are provided in thebase station, so that a downstream signal propagating through a backboneoptical fiber can be amplified with light from the LD 2, and an upstreamsignal propagating through the backbone and branch optical fibers can beamplified with light from the LD 3;

FIG. 12 is a view showing the configuration of a PON system of theinvention in which LD 1 and LD 2 for amplification are provided in thebase station, so that upstream and downstream signals propagatingthrough a backbone optical fiber can be amplified;

FIG. 13 is a view showing the configuration of a PON system of theinvention in which two LD 1 and LD 2 for amplification are provided inthe base station, so that upstream and downstream signals propagatingthrough a backbone optical fiber can be amplified;

FIG. 14 is a perspective view showing the structure of a WDMF; and

FIG. 15 is a graph showing the conditions of the Raman amplificationregarding a wavelength with respect to optical power.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the invention will be described in detailwith reference to the accompanying drawings.

A. Optical Communications System

FIG. 1 is a block diagram showing an optical communications systemfurnished with an optical amplification function of the invention. Aportion forming the optical communications system in the stationbuilding is referred to as the base station, and a portion forming theoptical communications system in a relay station is referred to as thelocal station. The optical communications system includes a base station1, a local station 5, an optical branching station 6, and a subscriber'shome 7. The base station 1 and the local station 5 are connected usingan optical fiber 2. The optical fiber 2 uses a single mode fiber.

Each of a down transmission signal from the base station 1 to the localstation 5 and an up transmission signal from the local station 5 to thebase station 1 comprises packets.

The base station 1 is furnished with a function of receiving packetssent from an upper network (Internet or the like) and sending thesepackets to the local station 5 via the optical network, and a functionof receiving packets sent from the local station 5 and sending thesepackets to the upper network.

The base station 1 includes a media converter serving as a connectionend to the optical fiber, a layer 2 switch, a broadband access routerserving as a connection end to the upper network, etc.

The local station 5 includes a media converter to transmit/receive abroadband signal to/from the optical network, an optical line terminalOLT, etc.

The subscriber's home 7 includes a personal computer PC installed withinthe house, an optical network unit ONU that transmits/receives abroadband signal outgoing from/incoming to the personal computer PCto/from the optical network, etc.

To briefly describe operations of the optical communications system,down packets coming into the base station 1 from the upper network aresubjected to specific processing in the layer 2 switch of the basestation 1. These packets are transmitted to the optical network by wayof the media converter. A light signal transmitted to the opticalnetwork is transmitted to the local station 5, and the local station 5takes in the light signal to decode the packets.

Meanwhile, up packets transmitted from the local station 5 aretransmitted to the base station 1. In the base station 1, after thespecific processing in the layer 2 switch, the packets are transmittedtherefrom to the upper network via the broadband access router.

An encoding method of a signal transmitted from the base station 1adopts a method by which the signal is biased to neither the high levelnor the low level even when data remains in a 0- or 1-state for a longperiod. For example, the Manchester encoding method can be adopted,according to which when data indicates 0, the signal is inverted fromthe high level to the low level at the center of the bit, and when dataindicates 1, the signal is inverted from the low level to the high levelat the center of the bit. When the NRZ encoding method is adopted, thesame advantages can be obtained by using a scheme such as convertingdata by adding redundancy bits to the original data to avoid a sequenceof 1's or 0's.

The optical amplification function furnished to the optical network willnow be described.

FIG. 2 is a network configuration view showing a state where the mediaconverter in the base station 1 and the media converter in the localstation 5 are connected to each other. According to this configuration,a high-power laser diode for a signal (High LD) is provided to the mediaconverter in the base station 1, and an upstream signal from the localstation 5 to the base station 1 is amplified with light from this laserdiode.

The media converter in the base station 1 includes a laser diode for adownstream signal (High LD, transmission wavelength: 1.4 μm) and a photodiode for an upstream signal (PD, reception wavelength: 1.5 μm). Boththe High LD and the PD are connected to the optical fiber 2 by way of aWDMF (Wavelength Division Multiplexing Filter). A bandpass opticalfilter BPF that allows passing of a desired reception wavelength aloneis added to the PD.

As is shown in FIG. 14, the WDMF has a structure in which λ-shapedwaveguides 61 and 62 are provided to a dielectric substrate 60, and adielectric multi-layer film filter 63 is provided at the contact pointof the waveguides 61 and 62. Light having a wavelength λ1 propagatesthrough the waveguide 62 and is reflected at the contact point, whereaslight having a wavelength λ2 propagates through the waveguide 61 andpasses through the contact point. A range of the wavelength λ1 to bereflected and a range of the wavelength λ2 to be allowed to pass can beset with the design of the dielectric multi-layer film filter 63.

The media converter in the local station 5 includes a laser diode for anupstream signal (LD for a signal, transmission wavelength: 1.5 μm), aphoto diode for a downstream signal (PD, reception wavelength: 1.4 μm),a WDMF, and a BPF.

Light having a wavelength of 1.4 μm from the High LD in the base station1 passes through the WDMF, and is received at the PD in the localstation 5 via the optical fiber 2 by passing through the WDMF and theBPF of the media converter.

Light from the LD for a signal in the local station 5 passes through theWDMF, and goes into the media converter in the base station 1 via theoptical fiber 2. Light for this upstream signal is reflected on the WDMFin the base station 1, and received at the PD in the base station 1.

Because the light having a wavelength of 1.4 μm from the High LD in thebase station 1 has a wavelength about 0.1 μm shorter than light for anupstream signal having a wavelength of 1.5 μm, it is possible to amplifylight for an upstream signal having a wavelength of 1.5 μm in theoptical fiber 2.

It should be noted that by forming part of, for example, a 3-km-longportion of the optical fiber 2 on the base station side using a highnonlinearity fiber (HNLF) and forming the remaining portion using an SMF(Single Mode Fiber), upstream signal light can be amplified moreeffectively.

The configuration in FIG. 2 shows an example of power design. Given thatthe optical fiber 2 is 100-km long, then a 4-km-long portion closer tothe media converter in the base station is formed using an HNLF, and theremaining 96-km-long portion closer to the optical branching station isformed using an SMF.

Assume that a propagation loss in the HNLF is 0.7 dB/km at a wavelengthof 1.4 μm and 0.5 dB/km at a wavelength of 1.5 μm. Assume that apropagation loss in the SMF is 0.4 dB/km at a wavelength of 1.4 μm and0.2 dB/km at a wavelength of 1.5 μm.

(Downstream Signal)

Optical power of High LD: 26 dBm

Transmission loss in WDMF: 1 dB

Propagation loss in HNLF: 0.7 dB/km×4 km=2.8 dB

Propagation loss in SMF: 0.4 dB/km×96 km=38.4 dB

Transmission loss in WDMF: 1 dB

Transmission loss in BPF: 1 dB

In the case specified as above, reception optical power at the mediaconverter in the local station is −18.2 dBm.

(Upstream Signal)

Optical power of LD for signal: 0 dBm

Transmission loss in WDMF: 1 dB

Propagation loss in SMF: 0.2 dB/km×96 km=19.2 dB

Propagation loss in HNLF: 0.5 dB/km×4 km=2.0 dB

Raman gain in SMF: 1.2 dB→half, 0.6 dB

Raman gain in HNLF: 11.6 dB→half, 8.8 dB

Transmission/reflection loss in WDMF: 1 dB

Transmission loss in BPF: 1 dB

When the Raman gain in the optical fiber 2 is ignored, reception powerof an upstream signal received at the PD in the base station is −24.2dBm.

Because 25 dBm of down optical power is pumped into the optical fiber 2,mathematically, the Raman gain in the high nonlinearity portion of theoptical fiber 2 is 11.6 dB, and the Raman gain in the SMF portion is 1.2dB. However, the High LD of the media converter in the base station doesnot constantly emit light. Although a 1000BASE-LX light signal adoptsthe NRZ encoding, because redundancy 2 bits are appended to the original8-bit data to convert the data so as to avoid a sequence of 0's or 1's,encoding takes place in such a manner that the number of 0-bits and thenumber of 1-bits are almost equal even in a silent state. Thelight-emitting time can be therefore deemed as nearly half. Then, theRaman gain in the HNLF of the optical fiber 2 is about half, 8.8 dB, andthe Raman gain in the SMF is about half, 0.6 dB. Hence, the receptionpower at the PD of the media converter in the base station is −14.8 dBm,which is a sum of −24.2 dBm and (8.8+0.6) dB. This is a level that themedia converter in the base station can receive with allowance.

B. PON System

FIG. 3 is a block diagram showing a PON system with the opticalamplification function of the invention. A portion forming the PONsystem in the station building is referred to as the base station, and aportion forming the PON system in the subscriber's home is referred toas the local station. The PON system includes a base station 1, plurallocal stations 5, and optical branching stations (referred to also asthe remote nodes) 3. The base station 1 and each optical branchingstation 3 are connected using a single backbone optical fiber 2, andeach optical branching station 3 and plural local stations 5 areconnected individually using branch optical fibers 4. The backboneoptical fiber 2 and the branch optical fibers 4 are collectivelyreferred to as the optical fiber. The optical fiber uses a single modefiber.

Each of a down transmission signal from the base station 1 to the localstation 5 and an up transmission signal from the local station 5 to thebase station 1 comprises packets.

The base station 1 is furnished with a function of receiving packetssent from an upper network (Internet or the like) and sending thesepackets to the local station 5 via the optical network, and a functionof receiving packets sent from the local station 5 and sending thesepackets to the upper network.

The base station 1 includes an optical line terminal OLT serving as aconnection end to the optical fiber, a layer 2 switch, a broadbandaccess router serving as a connection end to the upper network, etc.

The local station 5 includes a personal computer PC installed within thehouse, an optical network unit ONU that transmits/receives a broadbandsignal outgoing from/incoming to the personal computer PC to/from theoptical network, etc.

To briefly describe operations of the PON system, down packets cominginto the base station 1 from the upper network are subjected to specificprocessing in the layer 2 switch of the base station 1. These packetsare transmitted to the optical network by way of the optical lineterminal (OLT) A light signal transmitted to the optical network isbranched in the optical branching station 3, and transmitted to part orall of the local stations 5 connected to the optical branching station3. A local station 5 whose address coincides with the address of thetransmission destination takes in the light signal and decodes thepackets.

Meanwhile, up packets transmitted from the local station 5 aretransmitted to the base station 1 by way of the optical branchingstation 3. In the base station 1, after the specific processing in thelayer 2 switch, the packets are transmitted therefrom to the uppernetwork via the broadband access router.

An encoding method of a signal transmitted from the base station 1adopts a method by which the signal is biased to neither the high levelnor the low level even when data remains in a 0- or 1-state for a longperiod. For example, the Manchester encoding method can be adopted,according to which when data indicates 0, the signal is inverted fromthe high level to the low level at the center of the bit, and when dataindicates 1, the signal is inverted from the low level to the high levelat the center of the bit. When the NRZ encoding method is adopted, thesame advantages can be obtained by using a scheme such as convertingdata by adding redundancy bits to the original data to avoid a sequenceof 1's or 0's.

The configuration to achieve the optical amplification functionfurnished to the optical network will now be described.

FIG. 4 is a network configuration view showing a state where the opticalline terminal OLT in the base station 1, the optical branching station3, and the optical network unit ONU in the local station 5 areinterconnected. According to this configuration, an upstream signal fromthe optical branching station 3 to the base station 1 is amplified byproviding a high-power laser diode for a signal (High LD) in the OLT.

The optical line terminal OLT includes a laser diode for a downstreamsignal (High LD, transmission wavelength: 1.4 μm) and a photo diode foran upstream signal (PD, reception wavelength: 1.5 μm). Both the High LDand the PD are connected to a backbone optical fiber 2 by way of a WDMF(Wavelength Division Multiplexing Filter).

As is shown in FIG. 14, the WDMF has a structure in which λ-shapedwaveguides 61 and 62 are provided to a dielectric substrate 60, and adielectric multi-layer film filter 63 is provided at the contact pointof the waveguides 61 and 62. Light having a wavelength λ1 propagatesthrough the waveguide 62 and is reflected at the contact point, whereaslight having a wavelength λ2 propagates through the waveguide 61 andpasses through the contact point. A range of the wavelength λ1 to bereflected and a range of the wavelength λ2 to be allowed to pass can beset with the design of the dielectric multi-layer film filter 63.

The optical network unit ONU in the local station 5 includes a laserdiode for an upstream signal (LD for a signal, transmission wavelength:1.5 μm) and a photo diode for a downstream signal (PD, receptionwavelength: 1.4 μm).

The optical branching station 3 includes a star coupler, coupling thebackbone optical fiber 2 to the branch optical fibers 4, for opticalmultiplexing and demultiplexing.

Light having a wavelength of 1.4 μm from the High LD in the base station1 passes through the WDMF, and goes into the optical branching station 3via the backbone optical fiber 2, in which the light is demultiplexed toplural (for example, 32) wavelengths by means of the star coupler. Thebeams of demultiplexed light propagate through the respective branchfibers 4, and are received at PD's of the optical network units ONU's inthe respective local stations 5.

Light from the LD for a signal in the local station 5 travels throughthe branch optical fiber 4 and goes incident on the optical branchingstation 3, in which light is multiplexed by means of the star coupler.The multiplexed light goes into the optical line terminal OLT in thebase station 1 via the backbone optical fiber 2. Light for this upstreamsignal is reflected on the WDMF in the OLT, and received at the PD inthe base station 1.

Because the light having a wavelength of 1.4 μm from the High LD in thebase station 1 has a wavelength about 0.1 μm shorter than light for anupstream signal having a wavelength of 1.5 μm, it is possible to amplifylight for an upstream signal having a wavelength of 1.5 μm in thebackbone optical fiber 2.

It should be noted that by forming, for example, a 3-km-long portion ofthe backbone optical fiber 2 on the base station side using a highnonlinearity fiber and forming the remaining portion using an SMF(Single Mode Fiber), upstream signal light can be amplified moreeffectively.

When the Raman amplification is used, light for a signal with high poweris necessary, and safety has to be concerned. In this configuration,however, because amplifying light is attenuated by the transmission pathand the star coupler, power of light for a signal in the subscriber'shome and the ONU, in and with which a physical contact by the generalsubscriber is highly likely, is attenuated satisfactorily. The safetyconcerns are therefore unnecessary, or simple concerns are sufficient.

The configuration in FIG. 4 shows an example of power design. Given thatthe backbone optical fiber 2 is 12 km long, then a 3-km-long portioncloser to the OLT is formed using a high nonlinearity fiber, and theremaining 9-km-long portion closer to the optical branching station isformed using an SMF. The branch optical fiber 4 is 4 km long.

(Downstream Signal)

Optical power of High LD in OLT: 24 dBm

Transmission loss in WDMF: 1 dB

Propagation loss in high nonlinearity backbone optical fiber 2:0.7 dB/km×3 km=2.1 dB

Propagation loss in SMF backbone optical fiber 2:0.4 dB/km×9 km=3.6 dB

Multiplexing/demultiplexing loss in star coupler:18.5 dB

Propagation loss in branch optical fiber 4:0.4 dB/km×4 km=1.6 dB

Transmission loss in WDMF: 1 dB

(Upstream Signal)

Optical power of LD for signal in ONU: 0 dBm

Transmission loss in WDMF: 1 dB

Propagation loss in branch optical fiber 4:0.2 dB/km×4 km=0.8 dB

Multiplexing/demultiplexing loss in star coupler:18.5 dB

Propagation loss in SMF backbone optical fiber 2:0.2 dB/km×9 km=1.8 dB

Raman gain in SMF backbone optical fiber 2:0.75 dB→half, 0.4 dB

Propagation loss in high nonlinearity backbone optical fiber 2:0.5 dB/km×3 km=1.5 dB

Raman gain in high nonlinearity backbone optical fiber 2:6.8 dB→half, 4.6 dB

Transmission/reflection loss in WDMF: 1 dB

In the case specified as above, at a point at which upstream signallight from the ONU passes through the star coupler by propagatingthrough the branch optical fiber 4, signal power is −20.3 dBm.

When the Raman gain in the backbone optical fiber 2 is ignored,reception power of an upstream signal received at the PD in the basestation is −24.6 dBm.

Because −23 dBm of down optical power is pumped into the backboneoptical fiber 2, mathematically, the Raman gain in the high nonlinearityportion of the backbone optical fiber 2 is 6.8 dB, and the Raman gain inthe SMF portion is 0.75 dB. However, the High LD in the OLT does notconstantly emit light. Although a 1000BASE-LX light signal adopts theNRZ encoding, because redundancy 2 bits are appended to the original8-bit data to convert the data so as to avoid a sequence of 0's or 1's,encoding takes place in such a manner that the number of 0-bits and thenumber of 1-bits are almost equal even in a silent state. Thelight-emitting time can be therefore deemed as nearly half. Then, theRaman gain in the high nonlinearity portion of the backbone opticalfiber 2 is about half, 4.6 dB, and the Raman gain in the SMF portion isabout half, 0.4 dB. Hence, the reception power at the PD in the OLT inthe base station is −19.6 dBm as a result of the addition of (4.6+0.4)dB, which is the gain in the backbone optical fiber 2. This is a levelthat the OLT can receive with allowance.

The reception power at the ONU when light from the High LD in the OLTreaches the ONU is −3.8 dBm. This is power at a safe level when thesubscriber has a physical contact.

FIG. 5 is a network configuration view showing a state where the opticalline terminal OLT in the base station 1, the optical branching station3, and the optical network unit ONU in the local station 5 areinterconnected. According to this configuration, a high-power laserdiode for a signal (High LD, transmission wavelength: 1.4 μm), andplural photo diodes for an upstream signal(PD 1 through PD N, receptionwavelength: 1.5 μm band) are provided in the OLT. Further, an AWG thatsubjects upstream signal light coming into the OLT to wavelengthdivision is provided. Both the AWG and the High LD are connected to abackbone optical fiber 2 by way of a WDMF.

The optical branching station 3 is provided with the WDMF and the AWG.The WDMF reflects light having a wavelength of 1.4 μm from the High LDto be supplied to the star coupler. The star coupler sends downstreamsignal light to the respective ONU's via the branch optical fibers 41.The AWG multiplexes upstream signals propagating through the branchoptical fibers 42 and sends the multiplexed signal to the backboneoptical fiber 2.

Light having a wavelength of 1.4 μm from the High LD in the base station1 passes through the WDMF, and goes into the optical branching station 3via the backbone optical fiber 2. The light is then reflected on theWDMF and demultiplexed to plural (for example, 32) wavelengths by meansof the star coupler. The beams of demultiplexed light then travelthrough the respective branch optical fibers 41, and are received at thePD's in the optical network unit ONU's in the respective local stations5.

Light having a wavelength of 1.5 μm band from the LD for a signal in theONU in the local station 5 goes incident on the optical branchingstation 3 via the branch optical fiber 42. The light is then subjectedto wavelength division multiplexing (WDM) in the AWG, after which itpasses through the WDMF, and goes into the OLT in the base station 1 bypropagating through the backbone optical fiber 2. Light of this upstreamsignal is reflected on the WDMF in the OLT, and is further demultiplexedin the AWG according to wavelengths to be received at any of the PD 1through PD N in the base station 1.

Because light having a wavelength of 1.4 μm from the High LD in the basestation 1 has a wavelength about 0.1 μm shorter than light for anupstream signal having a wavelength of 1.5 μm band, it is possible toamplify light for an upstream signal having a wavelength of 1.5 μm bandin the backbone optical fiber 2.

Further, because this configuration uses the AWG having a small losswhen multiplexing and demultiplexing upstream light signals, power ofthe LD for a signal in the ONU can be lowered. This makes it easy toensure the safety in the subscriber's home and the ONU, in and withwhich a physical contact by the subscriber is highly likely.

The configuration in FIG. 5 shows an example of power design. Given thatthe backbone optical fiber 2 is 20 km long, then a 3-km-long portioncloser to the OLT is formed using a high nonlinearity fiber, and theremaining 17-km-long portion closer to the optical branching station isformed using an SMF. The branch optical fibers 41 and 42 are 4 km long.

(Downstream Signal)

Optical power of High LD in OLT: 24 dBm

Transmission loss in WDMF: 1 dB

Propagation loss in high nonlinearity backbone optical fiber 2:0.7 dB/km×3 km=2.1 dB

Propagation loss in SMF backbone optical fiber 2:0.4 dB/km×17 km=6.8 dB

Multiplexing/demultiplexing loss in star coupler:18.5 dB

Propagation loss in branch optical fiber 41,42:0.4 dB/km×4 km=1.6 dB

Transmission loss in WDMF: 1 dB

(Upstream Signal)

Optical power of LD for signal in ONU: 0 dBm

Transmission loss in WDMF: 1 dB

Propagation loss in branch optical fiber 41,42:0.2 dB/km×4 km=0.8 dB

Multiplexing/demultiplexing loss in AWG: 6 dB

Propagation loss in SMF backbone optical fiber 2:0.2 dB/km×17 km=3.4 dB

Raman gain in SMF backbone optical fiber 2:0.84 dB→half, 0.4 dB

Propagation loss in high nonlinearity backbone optical fiber 2:0.5 dB/km×3 km=1.5 dB

Raman gain in high nonlinearity backbone optical fiber 2:6.8 dB→half, 4.6 dB

Multiplexing/demultiplexing loss in AWG: 6 dB

Transmission/reflection loss in WDMF: 1 dB

In the case specified as above, at a point at which upstream signallight from the ONU passes through the AWG by propagating through thebranch optical fiber 4, signal power is −6.8 dBm.

When the Raman gain in the backbone optical fiber 2 is ignored,reception power of an upstream signal received at the PD in the basestation is −19.7 dBm.

Because −23 dBm of down optical power is pumped into the backboneoptical fiber 2, mathematically, the Raman gain in the high nonlinearityportion of the backbone optical fiber 2 is 6.8 dB, and the Raman gain inthe SMF portion is 0.84 dB. However, the High LD in the OLT does notconstantly emit light. Because a 1000BASE-LX light signal is encoded insuch a manner that the number of 0-bits and the number of 1-bits arealmost equal even in a silent state, the light-emitting time can bedeemed as nearly half. Then, the Raman gain in the high nonlinearityportion of the optical fiber 2 is about half, 4.6 dB, and the Raman gainin the SMF portion is about half, 0.4 dB. Hence, the reception power atthe PD of the OLT in the base station is −14.7 dBm as a result of theaddition of (4.6+0.4) dB, which is the gain in the backbone fiber 2.This is a level that the OLT can receive with allowance.

The reception power at the ONU when light from the High LD in the OLTreaches the ONU is −7 dBm. This is power at a safe level when thesubscriber has a physical contact.

Assume that the Manchester encoding is used to encode a downstream lightsignal, and a signal propagates at a communication rate of 10 Mbpsthrough a 10-km-long optical fiber having an effective refractive indexof 1.46. In this instance, information of about 500 bits is present inthe optical fiber. Because the Manchester encoding is used for theencoding, it is possible to encode one bit or two bits using a set of acombination of an ON state and an OFF state in data to be encoded. Thismeans that 250 to 500 sets of a combination of an ON state and an OFFstate are present in the optical fiber. Because about a half of the bitsare in the ON state and about the other half of the bits are in the OFFstate, it is possible to obtain about half the gain through the Ramanamplification for the entire optical fiber.

In the 1000BASE-LX, 8-bit information is converted to 10 bits byproviding redundancy in the physical layer for communications. At leastthe ON state is present twice and the OFF state is present twice in thisencoding with a few exceptions, and the codes are aligned in such amanner that the ON states and the OFF states are almost on halves.Hence, in the 1000BASE-LX, although it depends on the preceding orsucceeding information, it is thought that at least two sets of acombination of an ON state and an OFF state are necessary to encode8-bit information with a few exceptions. Given 1 M bits/sec as atransmission rate, then 8-bit information occupies about 1.6 m of theoptical fiber, and one set of a combination of an ON state and an OFFstate is thought to occupy about 0.8 m or less.

FIG. 6 is a network configuration view showing a state where the opticalline terminal OLT in the base station 1, the optical branching station3, and the optical network unit ONU in the local station 5 areinterconnected. According to this configuration, an upstream signal fromthe optical branching station 3 to the base station 1 is amplified byproviding a laser diode (LD) for amplification in the OLT.

The optical line terminal OLT in the base station 1 includes a laserdiode for a downstream signal (LD for a signal, transmission wavelength:1.3 μm), a laser diode for amplification of an upstream signal (LD foramplification, transmission wavelength: 1.4 μm), and a photo diode foran upstream signal (PD, reception wavelength: 1.5 μm). Both the LD foramplification and the PD are connected to a backbone optical fiber 22 byway of a WDMF (Wavelength Division Multiplexing Filter).

The optical network unit ONU in the local station 5 includes a laserdiode for an upstream signal (LD for a signal, transmission wavelength:1.5 μm) and a photo diode for a downstream signal (PD, receptionwavelength: 1.3 μm).

The optical branching station 3 includes a star coupler 31, for opticaldemultiplexing, to couple a backbone optical fiber 21 to branch opticalfibers 41, and a star coupler 32, for optical multiplexing, to couplebranch optical fibers 42 to the backbone optical fiber 22.

Light from the LD for a signal in the base station 1 goes into theoptical branching station 3 via the backbone optical fiber 21, and isdemultiplexed into plural (for example, 32) wavelengths by means of thestar coupler 31. The beams of demultiplexed light are connected to therespective branch optical fibers 41 and received at the PD's in therespective local stations 5.

Light from the LD for a signal in the local station 5 goes incident onthe optical branching station 3 via the branch optical fiber 42, and ismultiplexed by means of the star coupler 32. The multiplexed light goesinto the optical line terminal OLT in the base station 1 via thebackbone optical fiber 22. Light of this upstream signal is reflected onthe WDMF in the OLT and received at the PD in the base station 1.Meanwhile, light having a wavelength of 1.4 μm irradiated from the LDfor amplification in the base station 1 passes through the WDMF, andpropagates through the backbone optical fiber 22. Further, it isdemultiplexed by means of the star coupler 32 and the beams ofdemultiplexed light propagate through the branch optical fibers 42.Because the light having a wavelength of 1.4 μm has a wavelength about0.1 μm shorter than light for an upstream signal having a wavelength of1.5 μm, it is possible to amplify light for an upstream signal having awavelength of 1.5 μm during propagation.

It should be noted that by forming, for example, a 3-km-long portion ofthe backbone optical fiber 22 on the station side using a highnonlinearity fiber and forming the remaining portion using an SMF,upstream signal light can be amplified more effectively.

When the Raman amplification is used, light for amplification with highpower is necessary, and safety has to be concerned. In thisconfiguration, however, because amplifying light is attenuated by thetransmission path and the star coupler, power of light for amplificationin the subscriber's home and the ONU, in and with which a physicalcontact by the general subscriber is highly likely, is attenuatedsatisfactorily. The safety concerns are therefore unnecessary, or simpleconcerns are sufficient.

An example of power design will be described with reference to theconfiguration of FIG. 6.

Optical power of LD for signal in OLT: 0 dBm

Optical power of LD for amplification in OLT: 25 dBm

Loss in backbone optical fiber 21: 0.3 dB/km×6 km

Raman gain in backbone optical fiber 21:0.35 dB/km×6 km

Multiplexing/demultiplexing loss in star coupler 31:18.5 dB

Loss in optical branch fiber 41:0.2 dB/km×1 km

Optical power of LD for signal in ONU: −8 dBm

Transmission/reflection loss in WDMF: 0.5 dB

In the case specified as above, at a point at which upstream signallight from the ONU passes through the star coupler 31 by propagatingthrough the branch optical fiber 41, signal power is −26.7 dBm.

In a case where the LD for amplification in the OLT is omitted,reception power at the PD in the OLT of an upstream signal havingreached the base station is −29 dBm.

In a case where light is emitted from the LD for amplification in theOLT, reception power at the PD in the OLT in the base station is −26.9dBm as a result of the addition of 2.1 dB, which is the gain in thebackbone optical fiber 21.

In a case where light from the LD for amplification in the OLT isdemultiplexed by means of the star coupler 31 and the beams ofdemultiplexed light reaches the ONU's, reception power in each localstation is 4 dBm. This is a power at a safe level when the subscriberhas a physical contact.

FIG. 7 is a network configuration view showing a state where the opticalline terminal OLT in the base station 1, the optical branching station3, and the optical network unit ONU in the local station 5 areinterconnected. According to this configuration, in addition to theconfiguration of FIG. 6, a downstream signal from the base station 1 isamplified by providing an LD for amplification also in the opticalbranching station 3.

To describe only the additional configuration to FIG. 6, an LD foramplification (transmission wavelength: 1.2 μm) is provided in theoptical branching station 3, and amplifying light from the LD foramplification is connected to a down backbone optical fiber 21 by way ofthe WDMF. Signal light from the OLT that propagates through the downbackbone optical fiber 21 is reflected on the WDMF, and goes into thestar coupler 31. Meanwhile, light for amplification irradiated from theLD for amplification in the base station 1 passes through the WDMF andpropagates through the backbone optical fiber 21 between the basestation 1 and the optical branching station 3. Because the light foramplification having a wavelength of 1.2 μm has a wavelength about 0.1μm shorter than light for a downstream signal having a wavelength of 1.3μm, it is possible to amplify light for a downstream signal duringpropagation.

In this embodiment, an LD for amplification is provided in the opticalbranching station; however, another station that serves as neither anOLT nor an optical branching station may be prepared, so that LD's foramplification are provided concentrically therein. In this case, forexample, when ONU's are concentrated in a local area far from the OLTand distances among the optical branching stations in this area areshort, the need to provide an LD for amplification in each opticalbranching station can be eliminated, which can in turn save the costs.

FIG. 8 is a network configuration view showing a state where the opticalline terminal OLT in the base station 1, the optical branching station3, and the optical network unit ONU in the local station 5 areinterconnected. According to this configuration, both an upstream signalto the base station 1 and a downstream signal from the base station 1can be amplified by merely providing a single LD for amplification inthe optical line terminal OLT in the base station 1.

The configuration of the optical line terminal OLT in the base station 1is completely identical with the configuration described with referenceto FIG. 6 and FIG. 7. However, it is different in that a transmissionwavelength of the LD for amplification is 1.2 μm.

Two WDMF's are provided in the optical branching station 3. One WDMFareflects light from the LD for amplification in the OLT for the light tobe inputted into the other WDMFb. The light inputted into the WDMFbreaches the OLT via a down backbone optical fiber 21. Because awavelength of 1.2 μm of light for amplification is about 0.1 μm shorterthan a wavelength of 1.3 μm for a downstream signal, light for adownstream signal can be amplified during propagation. Light having awavelength of 1.3 μm from the LD for a signal in the base station 1 isalso reflected on the WDMFb to go into the star coupler 31.

According to this configuration, by allowing light from the LD foramplification in the base station 1 to travel through both the up anddown backbone fibers by way of the WDMFa and WDMFb in the opticalbranching station 3, it is possible to amplify a downstream signal fromthe base station 1. This enables the LD for amplification in the basestation 1 to supply up amplifying light, which in turn enablesdownstream signal light to be amplified while omitting power supply fromthe optical branching station 3.

By setting the wavelengths of both upstream signal light and downstreamsignal light to 1.3 μm, it is possible to amplify both the upstream anddownstream signals efficiently using a single LD for amplification.

In this embodiment, two WDMF's and two star couplers are prepared in theoptical branching station. However, when AWG's are used instead of thestar couplers, the same advantages can be achieved by connecting theWDMF to the AWG corresponding to a wavelength of the light foramplification.

FIG. 9 shows the configuration of a PON system including an additionalconfiguration to the configuration of FIG. 7, in which an upstream lightsignal from the optical network unit ONU in the local station 5 to theoptical line terminal OLT in the base station 1 is amplified with lightfrom an LDb for amplification provided in the optical branching station3.

To describe only the additional configuration to FIG. 7 alone, an LDbfor amplification (transmission wavelength: 1.2 Mm) is provided in theoptical branching station 3, and amplifying light from the LDb foramplification goes into the star coupler 32 by way of the WDMF. Thelight is then demultiplexed and the beams of demultiplexed light travelthrough the branch optical fibers 42 down to the respective localstations 5. An upstream signal (transmission wavelength: 1.3 μm) fromthe LD for a signal in the ONU is amplified with the amplifying light inthe branch optical fiber 42 before it reaches the optical branchingstation 3.

In this embodiment, the WDMF's and the 1:N star couplers are used;however, the same advantages can be achieved using 2:N star couplersalone. In this case, although optical power is reduced to half, both thecost and the size can be reduced because the WDMF's can be omitted.

In an example as follows, a single mode optical fiber that enablestwo-way propagation of a light signal is used.

FIG. 10 is a network configuration view showing a state where theoptical line terminal OLT in the base station 1 and the optical networkunit ONU in the local station 5 are connected to each other. Accordingto this configuration, by providing LD 2 and LD 3 for amplification inthe OLT, a downstream signal propagating through the backbone opticalfiber 2 between the base station 1 and the optical branching station 3is amplified with light from the LD 2, and an upstream signalpropagating through the backbone optical fiber 2 between the basestation 1 and the optical branching station 3 as well as the branchoptical fiber 4 between the optical branching station 3 and the localstation 5 is amplified with light from the LD 3.

The optical line terminal OLT in the base station 1 includes a laserdiode for a downstream signal (LD 1 for a signal, transmissionwavelength: 1.5 μm), a laser diode for amplification of a downstreamsignal (LD 2 for amplification, transmission wavelength: 1.4 μm), alaser diode for amplification of an upstream signal (LD 3 foramplification, transmission wavelength: 1.2 μ), a photo diode (PD,reception wavelength: 1.3 μm), and WDMFa through WDMFc. Light from theLD 2 for amplification is reflected on the first WDMFa, and is thenreflected on the third WDMFc, so that it propagates through the backboneoptical fiber 2 down to the optical branching station 3. Light from theLD 3 for amplification passes through the second WDMFb and the thirdWDMFc, and propagates through the backbone optical fiber 2 down to theoptical branching station 3.

A fiber Bragg grating FBG 34 of a band elimination type is inserted inthe optical branching station 3. This fiber Bragg grating reflects lighthaving a wavelength of 1.4 μm and transmits light having any otherwavelength. Hence, light having a wavelength of 1.4 μm from the LD 2 foramplification is reflected and returns to the base station 1. Lighthaving a wavelength of 1.5 μm from the LD 1 for a signal is thusamplified with light having a wavelength of 1.4 μm from the LD 2 foramplification that returns while the light is propagating through thebackbone optical fiber 2. This enables the LD 2 for amplification in thebase station 1 to supply up amplifying light, which in turn enablesdownstream signal light to be amplified while omitting the power supplyfrom the optical branching station 3.

Light having a wavelength of 1.2 μm from the LD 3 for amplificationpasses through the FBG 34, and is then demultiplexed by means of thestar coupler 33 functioning as an optical multiplexer/demultiplexer. Thebeams of demultiplexed light travel through the branch optical fibers 4down to the respective local stations 5.

The optical network unit ONU in the local station 5 includes a laserdiode for an upstream signal (LD for a signal, transmission wavelength:1.3 μm), a photo diode for a downstream signal (PD, receptionwavelength: 1.5 μm), and a WDMF. A downstream signal propagating throughthe branch optical fiber 4 is reflected on the WDMF and transmitted tothe PD. Light from the LD for a signal passes through the WDMF andpropagates through the branch optical fiber 4 in an upward direction.

Because the wavelength of upstream signal light from the LD for a signalis 1.3 μm, and the wavelength of light for down amplification from theLD 3 for amplification is 1.2 μm, the upstream signal light from the LDfor a signal is amplified while propagating through the branch opticalfiber 4 between the optical branching station 3 and the local station 5,and is also amplified while propagating through the backbone opticalfiber 2 between the base station 1 and the optical branching station 3.

FIG. 11 is a network configuration view showing a state where theoptical line terminal OLT in the base station 1 and the optical networkunit ONU in the local station 5 are connected to each other. Accordingto this configuration, by providing LD 2 and LD 3 for amplification inthe base station 1, a downstream signal propagating through the backboneoptical fiber 2 between the base station 1 and the optical branchingstation 3 is amplified with light from the LD 2, while an upstreamsignal propagating through the backbone optical fiber 2 between the basestation 1 and the optical branching station 3 as well as the branchoptical fiber 4 between the branching station 3 and the local station 5is amplified with light from the LD 3.

Differences from FIG. 10 are that the transmission wavelength of thelaser diode LD 1 for a downstream signal is 1.3 μm, the receptionwavelength of the photo diode PD is 1.5 μm, the transmission wavelengthof the LD 2 for amplification is 1.2 μm, and the transmission wavelengthof the LD 3 for amplification is 1.4 μm, and that the WDMFd, a fiberBragg grating FBG 34 of a band reflection type, and an optical fiber 35that connects these elements are provided in the optical branchingstation 3. The WDMFd reflects light having a wavelength of 1.2 μm andtransmits light having any other wavelength. The fiber Bragg gratingFBG34 allows light having a wavelength of 1.2 μm that was reflected onthe WDMFd to undergo total reflection.

Hence, light having a wavelength of 1.2 μm from the LD 2 foramplification returns to the WDMFd, and returns to the base station 1 bypropagating through the backbone optical fiber 2. Light having awavelength of 1.3 μm from the LD 1 for a signal is thus amplified withlight having a wavelength of 1.2 μm from the LD 2 for amplification thathas returned while the light is propagating through the backbone opticalfiber 2. This enables the LD 2 for amplification in the base station 1to supply up amplifying light, which in turn enables downstream signallight to be amplified while omitting the power supply from the opticalbranching station 3.

Light having a wavelength of 1.4 μm from the LD 3 for amplification isallowed to pass through and demultiplexed by means of the star coupler33 functioning as an optical multiplexer/demultiplexer. The beams ofdemultiplexed light travel through the branch optical fibers 4 down tothe respective local stations 5.

Only the difference of the local station 5 from FIG. 10 is that thewavelengths are exchanged in such a manner that the transmissionwavelength of the LD for an upstream signal is 1.5 μm and the receptionwavelength of the PD for a downstream signal is 1.3 μm.

Upstream signal light having a wavelength of 1.5 μm from the LD for asignal is amplified with light having a wavelength of 1.4 μm from the LD3 for amplification while the upstream signal light is propagatingthrough the branch optical fiber 4 between the optical branching station3 and the local station 5, and it is also amplified with light having awavelength of 1.4 gm from the LD 3 for amplification while it ispropagating through the backbone optical fiber 2 between the basestation 1 and the optical branching station 3.

The FBG 34 may be replaced with reflection processing, such as metalfilm coating, applied onto the end face of the optical fiber 35 throughwhich reflected light from the WDMFd propagates. This allows lighthaving a wavelength of 1.2 μm that has been reflected on the WDMFd toundergo total reflection.

Also, in this embodiment, light for amplification is extracted in theWDMF preceding the star coupler. However, when an AWG is used as theoptical multiplexer/demultiplexer 33, the same advantages can beachieved by providing a device (FBG, an optical fiber whose end face isprocessed for total reflection to take place) that allows totalreflection to a port from which the light for amplification isextracted.

FIG. 12 is a network configuration view showing a state where theoptical line terminal OLT in the base station 1 and the optical networkunit ONU in the local station 5 are connected to each other. Accordingto this configuration, upstream and downstream signals propagatingthrough the backbone optical fiber 2 between the base station 1 and theoptical branching station 3 are amplified by providing LD 1 and LD 2 foramplification in the base station 1.

The optical line terminal OLT in the base station 1 includes eight laserdiodes for a signal (LD 1 through LD 8 for a signal, transmissionwavelength: 1.5 μm band), a laser diode for amplification of adownstream signal (LD 2 for amplification, transmission wavelength: 1.4μm), a laser diode for amplification of an upstream signal (LD 1 foramplification, transmission wavelength: 1.2 μm), eight photo diodes (PD1 through PD 8, reception wavelength: 1.3 μm band), two AWG's(Arrayed-Wavelength Gratings), and two WDMF's.

Eight transmission signals are subjected to wavelength divisionmultiplexing (WDM) in the AWG and propagate through the backbone opticalfiber. Reception signals are demultiplexed in the AWG according to thewavelengths and received at the respective PD's.

An optical fiber 23 is independently provided between the base station 1and the optical branching station 3.

The WDMF and the AWG are provided in the optical branching station 3.The WDMF reflects light having a wavelength of 1.4 μm from the LD 2 foramplification and transmits any other light. An AWG wave, a downstreamsignal propagating through the backbone optical fiber 2 aredemultiplexed according to the wavelengths and sent to the respectiveONU's via the branch optical fibers 4.

Operations according to this configuration will now be described. Lighthaving a wavelength of 1.4 μm from the LD 2 for amplification reachesthe optical branching station 3 via the independently provided opticalfiber 23. It is then reflected on the WDMF in the optical branchingstation 3 and returns to the base station 1 by propagating through thebackbone optical fiber 2 in the upward direction.

Light having a wavelength of 1.2 μm from the LD 1 for amplificationpasses through the two WDMF's and propagates through the backboneoptical fiber 2 in the downward direction.

Meanwhile, a light signal having a wavelength of 1.5 μm band emittedfrom any of the LD 1 through LD 8 for a signal (for example, LD 1 for asignal) in the base station 1 passes through the AWG and is reflected onthe WDMF to exit from the backbone optical fiber 2. During thispropagation, it is amplified with return light having a wavelength of1.4 μm from the LD 2 for amplification. This enables the LD 2 foramplification in the base station 1 to supply up amplifying light, whichin turn enables downstream signal light to be amplified while omittingthe power supply from the optical branching station 3.

Light having a wavelength of 1.3 μm that exits from the local station 5and reaches the optical branching station 3 passes through the AWG andthe WDMF in the optical branching station 3, and reaches the basestation 1 by propagating through the backbone optical fiber 2. It isamplified with light having a wavelength of 1.2 μm from the LD 1 foramplification while it is propagating through the backbone optical fiber2.

As has been described, both the upstream and downstream light signalscan be amplified with light from the LD 1 and LD 2 for amplification.

It is more effective to use a high nonlinearity fiber for the backboneoptical fiber 2 and an SMF for the other optical fiber 23.

FIG. 13 is a network configuration view showing a state where theoptical line terminal OLT in the base station i and the optical networkunit ONU in the local station 5 are connected to each other. Accordingto this configuration, as with FIG. 12, upstream and downstream signalspropagating through the backbone optical fiber 2 between the basestation 1 and the optical branching station 3 are amplified by providingLD 1 and LD 2 for amplification in the base station 1.

A difference from FIG. 12 is that instead of providing the WDMF in theoptical branching station 3, light from the LD 2 for amplification thathas propagated through the independently provided optical fiber 23 isallowed to go into the AWG from a branch of the AWG on the local station5 side in the same manner as light from the local station 5.

This enables light for amplification having a wavelength of 1.4 μm topropagate through the backbone optical fiber 2 between the opticalbranching station 3 and the base station 1 toward the base station 1. Itis thus possible to amplify light for a downstream signal having awavelength of 1.5 μm exiting from the base station 1. This enables theLD 2 for amplification in the base station 1 to supply up amplifyinglight, which in turn enables downstream signal light to be amplifiedwhile omitting the power supply from the optical branching station 3.

While the embodiments of the invention have been described, theimplementation of the invention is not limited to the embodiments above.For example, the ONU in the local station includes the LD for anupstream signal and the PD for a downstream signal in the embodimentsabove. However, the LD for an upstream signal may be omitted, so thatlight coming incident as a downstream signal is demultiplexed by meansof a 3 dB coupler, and modulation processing to change a wavelength (seeJapanese Unexamined Patent Publication No.2001-177505 A) is performed,so that the light can be used as upstream signal light. Alternatively,an optical filter may be provided in the preceding stage of the photodiode PD. In addition, various modifications within the scope of theinvention are possible.

1. An optical communications system in which a base station and a localstation are connected using an optical fiber, the optical communicationssystem being characterized in that a wavelength of a light source for asignal that generates downstream signal light is set to a wavelengthwith an effect of Raman amplifying an upstream light signal thatpropagates through the optical fiber, and an upstream light signaltransmitted between the base station and the local station is amplifiedin the optical fiber while the upstream light signal is propagatingthrough the optical fiber.
 2. The optical communications systemaccording to claim 1, wherein a high nonlinearity fiber is used for atleast part of the optical fiber.
 3. The optical communications systemaccording to claim 1 or 2, wherein light that is switched ON and OFF isused as the downstream signal light, and a modulation method, by whichan ON state and an OFF state transit even when coded data is a sequenceof 0's and the ON state and the OFF state transit even when the codeddata is a sequence of 1's, is used as a modulation method for thedownstream signal light.
 4. The optical communication system accordingto claim 3, wherein in the backbone optical fiber, a length of a portionwhere upstream signal light is amplified is of a distance longer than alength of the optical fiber corresponding to a set of the ON state andthe OFF state of the downstream signal light.
 5. The opticalcommunication system according to any of claims 1 through 4, wherein thebase station is provided with an optical filter used to select awavelength of light coming incident on a light-receiving element.
 6. APON (Passive Optical Network) system in which a base station and anoptical branching station equipped with a passive optical divider areconnected using a backbone optical fiber, and the optical branchingstation and plural local stations are connected individually usingbranch optical fibers, the PON system being characterized in that awavelength of a light source for a signal that generates downstreamsignal light is set to a wavelength with an effect of Raman amplifyingan upstream light signal that propagates through the backbone opticalfiber, and an upstream light signal transmitted between the base stationand each local station is amplified in the backbone optical fiber whilethe upstream light signal is propagating through the backbone opticalfiber.
 7. The PON system according to claim 6, wherein a highnonlinearity fiber is used for at least part of the backbone opticalfiber.
 8. The PON system according to claim 6 or 7, wherein light thatis switched ON and OFF is used as the downstream signal light, and amodulation method, by which an ON state and an OFF state transit evenwhen coded data is a sequence of 0's and the ON state and the OFF statetransit even when coded data is a sequence of 1's, is used as amodulation method for the downstream signal light.
 9. The PON systemaccording to claim 8, wherein in the backbone optical fiber, a length ofa portion where upstream signal light is amplified is of a distancelonger than a length of the backbone optical fiber corresponding to aset of the ON state and the OFF state of the downstream signal light.10. The PON system according to any of claims 6 through 9, wherein thelight source for a signal and an optical multiplexer/demultiplexer areprovided in the base station, and light for a signal is pumped into thebackbone optical fiber from the base station toward the opticalbranching station by way of the optical multiplexer/demultiplexer. 11.The PON system according to any of claims 6 through 10, wherein a starcoupler is used as the passive optical divider.
 12. The PON systemaccording to any of claims 6 through 10, wherein as the passive opticaldivider, a star coupler is used for the downstream signal light, and anAWG (Arrayed-Waveguide Grating) capable of multiplexing anddemultiplexing upstream signal light using a difference in wavelength isused for the upstream signal light.
 13. A PON (Passive Optical Network)system in which a base station and an optical branching station equippedwith a passive optical divider are connected using a backbone opticalfiber, and the optical branching station and plural local stations areconnected individually using branch optical fibers, the PON system beingcharacterized by comprising: a light source for amplification thatgenerates light for amplification having a wavelength with an effect ofamplifying a light signal propagating through an optical fiber(including a backbone optical fiber and a branch optical fiber, and thesame applies hereinafter); and an optical multiplexer/demultiplexer usedto pump the light for amplification into the optical fiber, wherein, inthe optical fiber, a light signal transmitted between the base stationand each local station is amplified while the light signal ispropagating through the optical fiber.
 14. The PON system according toclaim 13, wherein Raman amplification is used as a function ofamplifying a light signal, and the light for amplification propagates ina direction opposite to the signal light.
 15. The PON system accordingto claim 13 or 14, wherein a high nonlinearity fiber is used.
 16. ThePON system according to claim 13, wherein an erbium-doped fiber (EDF) isused as a function of amplifying the light signal, and the signal foramplification is in the same direction as the signal light.
 17. The PONsystem according to claim 13, wherein the light source for amplificationand the optical multiplexer/demultiplexer are provided in the basestation, and the light for amplification is pumped into the backboneoptical fiber from the base station toward the optical branchingstation.
 18. The PON system according to claim 13, wherein the lightsource for amplification and the optical multiplexer/demultiplexer areprovided in the optical branching station, and the light foramplification is pumped into the backbone optical fiber from the opticalmultiplexer/demultiplexer toward the base station.
 19. The PON systemaccording to claim 17, wherein a second opticalmultiplexer/demultiplexer, a third optical multiplexer/demultiplexer,and an optical path connecting the second opticalmultiplexer/demultiplexer and the third opticalmultiplexer/demultiplexer are provided in the optical branching station,the light for amplification that travels through a backbone opticalfiber for an upstream signal is extracted from the second opticalmultiplexer/demultiplexer to be supplied to the third opticalmultiplexer/demultiplexer via the optical path, and the light foramplification is pumped into a backbone optical fiber for a downstreamsignal from the third optical multiplexer/demultiplexer toward the basestation.
 20. The PON system according to claim 13, wherein the lightsource for amplification and the optical multiplexer/demultiplexer areprovided in the optical branching station, and the light foramplification is pumped into the branch optical fiber by way of thepassive optical divider toward the local station.
 21. The PON systemaccording to claim 13, wherein the light source for amplification andthe optical multiplexer/demultiplexer are provided in the base station,and the light for amplification is pumped into the backbone opticalfiber from the base station toward the optical branching station, and areflector that allows the light for amplification to undergo totalreflection to the backbone optical fiber is provided in the opticalbranching station.
 22. The PON system according to claim 13, wherein thelight source for amplification and the optical multiplexer/demultiplexerare provided in the base station, and the light for amplification ispumped into the backbone optical fiber from the base station toward theoptical branching station, a second optical multiplexer/demultiplexerand a reflector are provided in the optical branching station, and thelight for amplification that travels through the backbone optical fiberis extracted from the second optical multiplexer/demultiplexer, so thatthe light for amplification is allowed to undergo total reflection onthe reflector.
 23. The PON system according to claim 13, wherein theoptical multiplexer/demultiplexer is provided in the optical branchingstation; an optical fiber is provided between the base station and theoptical branching station besides the backbone optical fiber, and thelight source for amplification is provided in the base station, and thelight for amplification is supplied to the opticalmultiplexer/demultiplexer via the optical fiber, so that the light foramplification is pumped into the backbone optical fiber from the opticalmultiplexer/demultiplexer toward the base station.
 24. The PON systemaccording to any of claims 17 through 23, wherein a star coupler is usedas the passive optical divider.
 25. The PON system according to claim13, wherein an optical fiber is provided between the base station andthe optical branching station besides the backbone optical fiber, andthe light source for amplification is provided in the base station, sothat the light for amplification is pumped into one optical path of theoptical multiplexer/demultiplexer on the local station side via theoptical fiber toward the base station.
 26. The PON system according toany of claims 17 through 23 and 25, wherein an AWG (Arrayed-WaveguideGrating) capable of multiplexing and demultiplexing light usingdifferent wavelengths is used as the passive optical divider.